1. Field of the Invention
The present invention relates to device control method and apparatus having a power saving mode function to reduce power consumption of a device.
2. Related Background Art
In a prior art electronic apparatus including a main unit, a peripheral device and a device control apparatus for controlling the peripheral device, there has been proposed a device control apparatus in which a power and a clock signal by an oscillation device are supplied to the peripheral device and the main unit is set to a so-called power saving mode when the main unit is in an actuatable stand-by state and an access such as an operation command is not present for a predetermined time interval. In the power saving mode, the supply of the power and the clock signal to the peripheral device is shut down and a clock generator included in a control unit of a CPU (central processing unit) for controlling the peripheral device is stopped to set a state by a device leakage current (so-called sleep mode) in order to attain the power saving.
In this device control apparatus, when a call from a line connected to the main unit or a signal generated by a command relating to the start of operation by an operator is received in the power saving mode, it is assumed as a factor to release the power saving mode and the main unit is recovered to a stand-by state. In recovering to the stand-by state, the sleep mode of the control unit is released and the shut-down of the supply of the power and the clock signal are released by the control through a port.
However, in the prior art device control apparatus described above, when the factor to release the power saving mode occurs, the release of the shut-down of the supply of the power and the clock signal and the release of the sleep mode of the control unit are immediately conducted. Since a certain time period is required before the supply of the power and the clock signal are stabilized, the state of the power supply and the oscillation device connected to the peripheral device is unstable when the main unit is recovered to the stand-by state so that the peripheral device circuit may malfunction.